1. Field of the Invention
The present invention relates to a tracing control system for machining a workpiece while tracing a model surface with a tracer head and, more particularly, to a tracing control system which reduces the extent to which the tracer head bites into the model surface.
2. State of the Relevant Art
Generally, to perform on-line tracing operation at high speed with high accuracy requires that a workpiece be simultaneously machined by a tracing control system.
FIG. 6 of the accompanying drawings schematically shows a conventional tracing control system. In FIG. 6, a stylus 142 of tracer head 141 moves in contact with an outer surface of a model 143, and displacements, .epsilon.x, .epsilon.y, .epsilon.z of the stylus 142 along these respective axes are detected by the tracer head 141. More specifically, the tracer head 141 detects displacements of the stylus 142 and generates corresponding displacement signals whose magnitudes correspond to the forces to which the stylus 142 is subjected, resulting from its contact with the model 143. A combining circuit 131 calculates the combined displacement .epsilon. of the stylus 142 according to an equation: .epsilon.=[(.epsilon.x.sup.2 +.epsilon.y.sup.2 +.epsilon.z.sup.2).sup.1/2 ]. An adder 132 calculates the difference .DELTA..epsilon. between the combined displacement .epsilon. and a reference displacement .epsilon..sub.0. A normal speed signal generator 133 multiplies the difference .DELTA..epsilon. by a predetermined gain to generate a normal speed signal Vn, i.e., a signal indicative of the speed in a normal direction. A tangential speed signal generator 134 generators a tangential speed signal Vt, i.e., a signal indicative of the speed in a tangential direction, from the difference .DELTA..epsilon. and a command tracing speed.
A switching circuit 135 selects the displacements along the respective axes in a tracing plane that is commanded. A tracing direction calculating circuit 136 calculates the cosine (cos.theta.) and sine (sin.theta.) of a tracing direction, or angle .theta., at which the model 143 is traced, from the selected displacements. An axis speed signal generator 137 processes the signals Vn, Vt, cos.theta., sin.theta., generated from the displacement signals, into axis speed signals Vx, Vy, Vz for the corresponding tracing axis of the tracer head 141. The axis speed signals Vx, Vy, Vz are supplied to the tracer head 141 to move the stylus 142 on the surface of the model 143 and also to a cutter head 162 to move the cutter head 162 at the speeds represented by the axis speed signals Vx, Vy, Vz for thereby machining a workpiece 163.
In the conventional tracing control system which effects such a simultaneous machining process on the workpiece 163, the cutter head 162 and the tracer 141 are mechanically coupled to each other such that the relative positional relationship between the workpiece 163 and the cutter head 162 is controlled so as to be equal to the relative positional relationship between the model 143 and the tracer head 141. However, there is a certain limitation on the tracing speed of the tracer head 141, and if the tracing speed is excessively high, the cutter head 162 tends to bite into the workpiece 163 in a region where the profile of the model 143 abruptly changes.
The cutter head 162 is caused to bite into the workpiece 163 because the cutter head 162 and the tracer head 141 are in the same relative position, and when the stylus 142 overshoots beyond a reference distance with respect to the tracer head 141, the overshooting of the stylus 142 is reflected directly in the biting engagement of the cutter head 162 with the workpiece 163.
There has been developed a digitizer for solving the above problem. The digitizer reads, from time to time, the positions of a tracer head along the respective axes as digital positional data, and the model surface is linearly approximately based on the digital positional data. The positional data is processed to prevent a cutter head from biting into a workpiece, e.g., by temporarily interrupting the reading of positional data in a region where the profile of the model surface abruptly changes, thereby generating numerical control ("NC") data. The generated NC data is then applied to a computerized numerical control machine, which machines a workpiece with a cutter head.
Since the tracing process which uses the digitizer machines the workpiece after the model has been traced, however, the number of steps required is relatively large, and no efficient tracing operation can be effected.
The applicants have proposed in the application heil-219832 a tracing control system having a main motor for actuating a cutting machine as a whole, the main motor being driven in three axes of a first coordinate system. A tracer head is driven in three axes of a second coordinate system which, however, is associated with the respective three axes and coordinate system of the main motor. The driving of the tracer head is produced by an auxiliary motor. The main and auxiliary motors are controlled in relation to each other for reducing any biting engagement of a cutter head with a workpiece when it is machined simultaneously with the tracing of a model by the tracer head.
In the proposed tracing control system, the tracer head is moved ahead of the cutter head to prevent the cutter head from biting into the workpiece in a region where the profile of the model abruptly changes. As the speed of the racer head is reduced, the distance by which the tracer head moves ahead of the cutter head is also reduced, resulting in a corresponding reduction in the amount by which the cutter head bites into the workpiece. However, inasmuch as the distance, by which the tracer head moves ahead of the cutter head, is determined depending on the difference between delay time constants of the main and auxiliary motors, the tracing speed of the tracer head cannot be substantially increased. Furthermore, the amount by which the cutter head bites into the workpiece cannot be reduced beyond a certain limit, upon the occurrence of overshooting of the tracer head, because the displacements of the stylus along the respective axes cannot be increased beyond reference displacements.